1. Field of the Invention
The present invention relates to superconductive oxides whose critical temperatures are in a high temperature range, and more particularly the invention relates to single crystals of such oxides as La.sub.2-x A.sub.x CuO(A:Sr, Ba), Nd.sub.2-x Ce.sub.x CuO.sub.4, YBa.sub.2 CuO4, YBa.sub.2 ACu.sub.3 O.sub.7-x, BiSrCaCu.sub.2 O.sub.x and Tl.sub.2 Ba.sub.2 Ca.sub.2 CuO.sub.x and a manufacturing method thereof.
2. Description of the Prior Art
Since Dr. J. G. Bedonoz And Dr. K. A. Muller discovered in 1986 that even oxides could exhibit superconductive properties at elevated temperatures, the investigations of many superconductive oxides have been conducted throughout the world.
It has been known that certain types of oxides, e.g., La.sub.2-x A.sub.x CuO.sub.4 (A:Sr, Ba), Nd.sub.2-x CeCuO.sub.4, YBa.sub.2 Cu.sub.3 O.sub.7-x, BiSrCaCu.sub.2 O.sub.x and Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x exhibit superconductivity at higher critical temperatures than those of conventional metal type superconducting materials, although these oxides are extremely low in density of states of electrons.
The investigations of these oxides have mainly dealt with sintered materials and thin films and the investigations have considerably gone into detail with respect to the crystal structures and the relation between the chemical compositions and the critical temperatures.
However, there have been known no established principles on the superconducting mechanism of these superconductive oxides.
With practically all of the oxide superconducting materials which have been reported up to the date, their basic structure is composed of the perovskite lattice and they belong to the tetragonal system or the orthorhombic system and not to the cubic system differing from the metal or alloy superconducting materials. Consequently, it has been considered that the anisotropic information cannot be obtained from the physical properties of the sintered materials which are aggregates of polycrystals, that it is difficult to obtain the information in the thickness direction of the thin films and that it is difficult to construct the superconducting mechanism of these materials.
In order that the physical properties, e.g., the anisotropy of the magnetic and electric properties of any oxide can be measured precisely to obtain the anisotropy information and elucidate its superconductivity, a large and good-quality single crystal of the oxide is required and therefore there is a need for the growth of a single crystal which is good in quality and large in size.
Included among the single crystals of the oxides of the high-temperature superconducting materials which have been reported as succeeded in growing up to the data are LA--Sr--Cu--O group, Nd--Ce--Cu--O group, Y--Ba--Cu--O group, Bi--Ca--Ba--Cu--O group, etc. Since practically all of these materials are considered to be decomposed and molten compounds, it is impossible to use for the growing of single crystals such melting and solidifying methods as the pull method and the Bridgman method which have been used for manufaturing the ordinary oxide single crystals. The methods used mainly for the purpose include the flux method and the top seeded solution method which is a modification of the flux method, and an attempt employing the floating zone method on a single crystals of Bi-group has been reported by Takekawa et al [J. Crst. growth, 982(1988)9687].
Also, there has been a report on the eutectic composition crystals of La.sub.2 CuO.sub.4 and CuO as shown in Table 1 which will be described later [L. Trouilleux. G. Dhalenne and A. Revcolevschi: Cryst Growth, 91(1988)268].
The solvent used in the flux method is CuO, called as a self flux, in many instances and after a crystal has been grown, the flux and the formed crystal are separated mechanically, thus making it difficult to separate the solvent and the grown crystal from each other.
In the case of a single crystal of lanthanum-system La.sub.2-x A.sub.x CuO.sub.4 group, however, the crystal grown in the flux and sunk to the bottom of the crucible is taken up thus trying to separate it from the solvent.
In any case, crystals grown by the flux method are not so large in size and the flux is contained in any large-size crystals grown. Also, a thin planar crystal is generally grown along the c-axis.
The following Table 1 shows the sizes of the crystals grown, the solvents and growing methods used, the critical temperatures, etc., which have been reported up to the date with respect to the single crystals of lanthanum-group.
TABLE 1 ______________________________________ Examples of growing La.sub.2-x AxCuO.sub.4 (A:Sr, Ba) single crystal SOL- SIZE METHOD VENT OF CRYSTAL T(c) (K) .DELTA.Tc (K) ______________________________________ Flux Method Ref. 1 CuO 8 .times. 8 .times. 2 30 26.2 Ref. 2 25 .times. 20 .times. 5 25 13 Ref. 3 PbO 0.14 .times. 0.14 .times. 0.009 8.5-9.0 Broad Ref. 4 CuO 17 .times. 14 .times. 1 N.D. Ref. 5 CuO 4 .times. 7 .times. 0.1 13.4 Ref. 6 CuO 2 .times. 1 .times. 0.2 26 Top seeded solution method Ref. 7 CuO 18 mm.phi., 4 mm long N.D. Ref. 8 CuO, 25 .times. 25 .times. 5 &lt;4 Li.sub.4 B.sub.2 O.sub.5 or Na.sub.2 B.sub.2 O.sub.4 Ref. 9 CuO 8 .times. 8 .times. 12 &lt;5 Floating zone method Ref. 10 eutectic 7 mm.phi., several cm 35 long ______________________________________ Ref. 1: Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T. Murakami: J. Cryst. Growth, 85 (1987) 581. Ref. 2: S. Shamot: Solid State Commun., 66 (1988) 1151 Ref. 3: H. H. Wnag, U. Geiser, R. J. THorn, K. D. Carlson, M. A. Beno, M. R. Monaghan, T. J. Allen, R. B. Proksch, D. L. Stupka, W. K. Kwok, G. W. Crabtrree and J. M. Williams: Inoganic Chem. 26 (1987) 1190. Ref. 4,7: K. Oka and H. Unoki: Jpn. J. Appl. Phys., 26 (1987) L1590 Ref. 5: U. Kawbe, H. Hasegawa, T. Aita and T. Ishiba: Jpn. J. Phys., 26 Supplement3 (1987) 1135. Ref. 6: A. B. Bykov, L. N. Demyanets, N. D. Zakharov, B. Y. Kotyuzhanskii I. N. Makarenkp, O. K. Melnicov, V. M. Molchanov, L. A. Prozorova and L. V. Svistunov: Pisma. ZH. Eksp. Tecr. Fiz., 46 (1987) 19. Ref. 8: P. L. Picone, H. P. Jenessen and D. R. Gabbe: J. Gryst. Growth, 8 (1987) 576. Ref. 9: A. B. Bykov, L. N. Demianets, I. P. Zibron, G. V. Kanunnikov, O. K. Melnikov and S. M. Stishov: J. Cryst. Growth, 91 (1983) 302. Ref. 10: L. Trouilleux. g. Dhalenne ans A. Revcolevschi: J. Cryst. Growth 91 (1988) 268.
As will be seen from Table 1, the planar crystals were produced in practically all the cases as mentioned previously. The sizes of the crystals grown by the top seeded solution method were as considerable large as 25.times.25.times.5 mm but were very low in critical temperature. This is considered to be due to the fact that the solid solution Sr(Ba) was less than the raw material composition. Also, in the case according to the floating zone method shown in the last part of Table 1, although the crystal grown was large in size and was also somewhat higher in critical temperature than those of the other methods, the raw material composition was an eutectic composition including CuO and the crystal grown was an autectic crystal which could not be said to be a single phase crystal.
As described hereinabove, the conventional superconductive oxide crystals were either not so large in size or containing the flux in the case of the large ones and also they were each in the form of a thin planar crystal along the c-axis, thus making it difficult to precisely measure the physical properties of the oxides such as the anisotropy of the magnetic and electric properties to obtain the desired anisotropic information and also making it difficult to construct the superconducting mechanism of the single crystals.